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Greenland melting speeds up

13 November 2009, by Tamera Jones

The Greenland ice sheet is losing ice at an ever increasing rate, say scientists. What's more, they've shown for the first time that the loss of ice is split equally between what's happening on the surface - like ice melting and varying levels of snowfall - and icebergs calving off from the fronts of glaciers.

Isfjord, Greenland: calving icebergs from the largest outlet glacier in the country. The glacier doubled in speed during the late 1990s and is partly responsible for increased ice mass loss from Greenland.

Although scientists have noticed that the speed at which Greenland is losing ice has increased in recent years, until now they couldn't say exactly how much ice was being lost. The most recent estimates vary by a factor of two.

The researchers describe in the journal Science how they used two separate methods to find out how much ice Greenland lost between 2000 and 2008. A climate simulation let them calculate snowfall and surface melt on the ice sheet. Satellite data from 38 glacier drainage basins across Greenland gave the scientists an estimate of the rate of iceberg production over the same period.

GRACE

The team also analysed data from the US Gravity Recovery and Climate Experiment satellite (GRACE) which essentially weighs the ice sheet.

Results from the two completely independent methods were extremely similar. 'This gives us confidence when separating out the components,' says one of the study's co-authors, Professor Jonathan Bamber from the University of Bristol.

'What we can't say for sure is that it's purely down to anthropogenic warming. But we are seeing a trend that's a decade long, and it's bigger than the normal natural variability.'Professor Jonathan Bamber, University of Bristol

They found that between 2000 and 2008, Greenland lost around 1500 billion tons of ice, which is equivalent to 166 billion tons a year or 0.46 millimetres a year of global sea level rise. But in each year between 2006 and 2008, the ice sheet lost 273 billion tons of ice per year, equivalent to 0.75 millimetres of sea level rise.

'This is down to increased surface melting as well as glaciers speeding up in recent years,' says Bamber.

Sea level has been rising on average by around 3.2 millimetres per year since the 1990s, with the latest estimates suggesting that Greenland contributes 0.5 millimetres per year to sea-level rise. 'It's clear this number has been rapidly increasing and will be nearer one millimetre a year by the end of the decade if the trend continues unabated,' says Bamber.

In some parts of Greenland, like the wet southeast, calving icebergs were a more important part of the total ice loss than increased melting at the surface. But in the north, surface melting contributed much more to ice loss than accelerating glaciers.

'That was a surprise. We didn't expect surface processes to be more important, because in the last five or six years, people haven't taken so much notice of this. They've really focussed on accelerating glaciers,' says Bamber.

'What we can't say for sure is that it's purely down to anthropogenic warming. But we are seeing a trend that's a decade long, and it's bigger than the normal natural variability. From what we know about the mechanisms causing the changes, it looks like things will get worse, at least for the next few years. Beyond that, it's hard to say,' he adds.

The research group was made up of an international team of scientists from Utrecht University, the University of Bristol, the University of California, Irvine, the Jet Propulsion Laboratory, Pasadena, the Delft University of Technology and the Royal Netherlands Meteorological Institute.